Ferro Corporation: Confidential and Proprietary

Front Silver and Back Surface Field Metallization for

Achieving High Performance Crystalline Silicon Solar Cell

Ferro CorporationApril 14, 2010

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Ferro Corporation: Confidential and Proprietary

Key Parameters for High Performance Front Contact

Contact Resistance– Controlled through glass chemistry and metal powder

Front grid Resistance– Line width, line height and line resistivity

Diode quality– Penetration of silver crystallites into emitter– Size & distribution of silver crystallites

Aspect Ratio (Topic of Discussion)

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Ferro Corporation: Confidential and Proprietary

High Aspect Ratio - Ideas

Increase solids content in the front contact pastes– New pastes available at 88-90% (w/w) solids loading– 5% increase of solids, 33% reduction of organics

Double Printing ProcessStencil printingHot Melt printing

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“Print-on-Print” Objective

Two print process for front side (pilot scale)

Print 1 (Bottom Layer)– Finger pattern without bus bars– High aspect ratio

DryPrint 2 (Top Layer)– Full cell pattern with bus bars– Exact alignment needed

Dry

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Ferro Corporation: Confidential and Proprietary

“Print-on-Print” Properties

Print 1– High Solids Paste– Good Contact Resistance– ≤ 80 µm Line Width– ≥ 0.3 Aspect Ratio

Print 2– Moderate Solids Paste– Low Bulk Resistivity– ≤ 80 µm Line Width– Good Adhesion

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Paste Properties

NS164 (Print 2)– 86 - 90 % Solids – 80 ±10 Pa*s Viscosity– -0.6 ± 0.1 Slope– Bulk Resistivity

2.5 x10-6 Ω-cm

NS165 (Print 1)– 90 - 92 % Solids – 90 ±10 Pa*s Viscosity– -0.6 ± 0.1 Slope– Bulk Resistivity

3.2 x10-6 Ω-cm

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Ferro Corporation: Confidential and Proprietary

“Fired” NS164 (60 µm opening) over NS165 (80 µm opening)

Uniform lines with Aspect Ratio of > 0.33 was achieved

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Ferro Corporation: Confidential and Proprietary

Hot Melt Print

Aspect Ratio of0.48 achieved

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Ferro Corporation: Confidential and Proprietary

Hot Melt Process Reduces Process StepsWafer

Print Back Ag Contact

Dry

Print Front Finger Ag

Dry

Fire

Wafer

Print Front Contact

Print Back Contact Ag

Fire

Print Back Al

Print Back Al

Dry

Hot Melt

Print-on-Print

Hot Melt reduces process steps by50 % and eliminates 3 dryers

Print Front Bus Bar Ag

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Ferro Corporation: Confidential and Proprietary

Print-on-Print and Hot Melt

Hot MeltPositives

– 50 % less process steps– Possibility of better aspect ratio

Negatives– Need Hot Melt printer and

supporting equipment– Need optimization of print

process to achieve same line continuity as double print process

Print-on-PrintPositive

– Uses similar paste technology– Gives flexibility for different

layers• Bottom layer low contact

resistance• Top layer low bulk

resistance

Negatives– Doubles printing steps– Complex alignment process

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Ferro Corporation: Confidential and Proprietary

Conclusions

Hot Melt is a low cost high performance solution.

Ferro supports both Hot Melt and Print-on-Print process.

Ferro prefers Hot Melt process due to its advantages.

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Ferro Corporation: Confidential and Proprietary

High Performance Through Back Surface Field Al

High Performance Al– Thick BSF ( > 7 microns)– Back surface reflection

Seff vs. BSF Thickness

0500

100015002000250030003500400045005000

0 5 10 15 20 25 30

Wbsf (um)

Seff

(cm

/sec

)

Seff

Seff vs. BSF doping

1.E+02

1.E+03

1.E+04

1.E+05

1.E+16 1.E+17 1.E+18 1.E+19 1.E+20 1.E+21

BSF doping cm^-3)

Seff

(cm

/sec

)

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Ferro Corporation: Confidential and Proprietary

Effect of Firing Temperature on BSF

Higher the firing temperatureHigher is the amount of dissolved Si

This translates to thicker stronger doped BSF

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Ferro Corporation: Confidential and Proprietary

BSF vs. Thickness

Higher firing temperature increases thickness of BSF

900 C

990 C

960 C

940 C920 C

980 C970 C

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Ferro Corporation: Confidential and Proprietary

Voc vs firing profile

V oc vs. firing profile

0.6090.6120.6150.6180.6210.6240.6270.6300.633

890 910 930 950

F iring tem p ( C )

Voc (V)

220 belt speed

240 belt speed

Higher BSF ThicknessResults in Higher Voc.

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Ferro Corporation: Confidential and Proprietary

Effect of Firing Temperature on a Standard Cell Performance

19C Eff/9 zone T

14.114.414.7

1515.315.615.916.216.516.817.117.4

890 910 930 950

220 belt speed

240 belt speed

Efficiency

Eff vs. Firing Temperature Loss of Efficiency due to high contact resistance due to glass at interface

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Effect of Firing Temperature on Etched Front Contact

Increase in firing temperature causes growth ofsilver crystallites and glass layer at interface.This reduces the optimum cell firing temperature .

900

990

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Ferro Corporation: Confidential and Proprietary

Maximum Efficiency Improvement Through Back Al

6 mv (~1%) of Voc is not realized due to overfiring of Front.By optimizing Front Contact to fire at higher temperature additional efficiency gain can be realized from enhanced BSF.

Island growth can be controlledThrough glass chemistry

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Ferro Corporation: Confidential and Proprietary

Efficiency Improvement Concepts for Consideration

Front Contact

– 0.33 Aspect Ratio through two-step printing– 0.48 Aspect Ratio through Improved Hot Melt Technology

Back Surface Aluminum– Realization high strength and deeper BSF through increased firing

temperature.– Re-optimization of Front Contact to match the Aluminum

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Ferro Corporation: Confidential and Proprietary

Thank you for you attention!